1. Field of the Invention
This invention relates generally to endpoint detection in a chemical mechanical planarization process, and more particularly to endpoint detection using a raised detection window.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess metallization.
A chemical mechanical planarization (CMP) system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically made of a polyurethane material. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
In the prior art, CMP systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
FIG. 1A shows a cross sectional view of a dielectric layer 2 undergoing a fabrication process that is common in constructing damascene and dual damascene interconnect metallization lines. The dielectric layer 2 has a diffusion barrier layer 4 deposited over the etch-patterned surface of the dielectric layer 2. The diffusion barrier layer, as is well known, is typically titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN) or a combination of tantalum nitride (TaN) and tantalum (Ta). Once the diffusion barrier layer 4 has been deposited to the desired thickness, a copper layer 6 is formed over the diffusion barrier layer in a way that fills the etched features in the dielectric layer 2. Some excessive diffusion barrier and metallization material is also inevitably deposited over the field areas. In order to remove these overburden materials and to define the desired interconnect metallization lines and associated vias (not shown), a chemical mechanical planarization (CMP) operation is performed.
As mentioned above, the CMP operation is designed to remove the top metallization material from over the dielectric layer 2. For instance, as shown in FIG. 1B, the overburden portion of the copper layer 6 and the diffusion barrier layer 4 have been removed. As is common in CMP operations, the CMP operation must continue until all of the overburden metallization and diffusion barrier material 4 is removed from over the dielectric layer 2. However, in order to ensure that all the diffusion barrier layer 4 is removed from over the dielectric layer 2, there needs to be a way of monitoring the process state and the state of the wafer surface during its CMP processing. This is commonly referred to as endpoint detection. Endpoint detection for copper is performed because copper cannot be successfully polished using a timed method. A timed polish does not work with copper because the removal rate from a CMP process is not stable enough for a timed polish of a copper layer. The removal rate for copper from a CMP process varies greatly. Hence, monitoring is needed to determine when the endpoint has been reached. In multi-step CMP operations there is a need to ascertain multiple endpoints: (1) to ensure that Cu is removed from over the diffusion barrier layer; (2) to ensure that the diffusion barrier layer is removed from over the dielectric layer. Thus, endpoint detection techniques are used to ensure that all of the desired overburden material is removed.
Many approaches have been proposed for the endpoint detection in CMP of metal. The prior art methods generally can be classified as direct and indirect detection of the physical state of polish. Direct methods use an explicit external signal source or chemical agent to probe the wafer state during the polish. The indirect methods on the other hand monitor the signal internally generated within the tool due to physical or chemical changes that occur naturally during the polishing process.
Indirect endpoint detection methods include monitoring: the temperature of the polishing pad/wafer surface, vibration of polishing tool, frictional forces between the pad and the polishing head, electrochemical potential of the slurry, and acoustic emission. Temperature methods exploit the exothermic process reaction as the polishing slurry reacts selectively with the metal film being polished. U.S. Pat. No. 5,643,050 is an example of this approach. U.S. Pat. No. 5,643,050 and U.S. Pat. No. 5,308,438 disclose friction-based methods in which motor current changes are monitored as different metal layers are polished.
Another endpoint detection method disclosed in European application EP 0 739 687 A2 demodulates the acoustic emission resulting from the grinding process to yield information on the polishing process. Acoustic emission monitoring is generally used to detect the metal endpoint. The method monitors the grinding action that takes place during polishing. A microphone is positioned at a predetermined distance from the wafer to sense acoustical waves generated when the depth of material removal reaches a certain determinable distance from the interface to thereby generate output detection signals. All these methods provide a global measure of the polish state and have a strong dependence on process parameter settings and the selection of consumables. However, none of the methods except for the friction sensing have achieved some commercial success in the industry.
Direct endpoint detection methods monitor the wafer surface using acoustic wave velocity, optical reflectance and interference, impedance/conductance, electrochemical potential change due to the introduction of specific chemical agents. U.S. Pat. No. 5,399,234 and U.S. Pat. No. 5,271,274 disclose methods of endpoint detection for metal using acoustic waves. These patents describe an approach to monitor the acoustic wave velocity propagated through the wafer/slurry to detect the metal endpoint. When there is a transition from one metal layer into another, the acoustic wave velocity changes and this has been used for the detection of endpoint. Further, U.S. Pat. No. 6,186,865 discloses a method of endpoint detection using a sensor to monitor fluid pressure from a fluid bearing located under the polishing pad. The sensor is used to detect a change in the fluid pressure during polishing, which corresponds to a change in the shear force when polishing transitions from one material layer to the next. Unfortunately, this method is not robust to process changes. Further, the endpoint detected is global, and thus the method cannot detect a local endpoint at a specific point on the wafer surface. Moreover, the method of the 6,186,865 patent is restricted to a linear polisher, which requires an air bearing.
There have been many proposals to detect the endpoint using the optical reflectance from the wafer surface. They can be grouped into two categories: monitoring the reflected optical signal at a single wavelength using a laser source (such as, for example, 600 nm) or using a broad band light (such as, for example, 255 nm to 700 nm) source covering the full visible range of the electromagnetic spectrum. U.S. Pat. No. 5,433,651 discloses an endpoint detection method using a single wavelength in which an optical signal from a laser source is impinged on the wafer surface and the reflected signal is monitored for endpoint detection. The change in the reflectivity as the polish transfers from one metal to another is used to detect the transition. Unfortunately, the single wavelength endpoint detection has a problem of being overly sensitive to the absolute intensity of the reflected light, which has a strong dependence on process parameter settings and the selection of consummables. In dielectric CMP applications, such single wavelength endpoint detection techniques also have a disadvantage that it can only measure the difference between the thickness of a wafer but typically cannot measure the actual thickness of the wafer.
Broad band methods rely on using information in multiple wavelengths of the electromagnetic spectrum. U.S. Pat. No. 6,106,662 discloses using a spectrometer to acquire an intensity spectrum of reflected light in the visible range of the optical spectrum. In metal CMP applications, the whole spectrum is used to calculate the end point detection (EPD signal). Significant shifts in the detection signal indicate the transition from one metal to another.
A common problem with current endpoint detection techniques is that some degree of over-etching is required to ensure that all of the conductive material (e.g., metallization material or diffusion barrier layer 4) is removed from over the dielectric layer 2 to prevent inadvertent electrical interconnection between metallization lines. A side effect of improper endpoint detection or over-polishing is that dishing 8 occurs over the metallization layer that is desired to remain within the dielectric layer 2. The dishing effect essentially removes more metallization material than desired and leaves a dish-like feature over the metallization lines. Dishing is known to impact the performance of the interconnect metallization lines in a negative way, and too much dishing can cause a desired integrated circuit to fail for its intended purpose. In view of the foregoing, there is a need for endpoint detection systems and methods that improve accuracy in endpoint detection.
FIG. 1C shows a prior art belt CMP system 10 in which a pad 12 is designed to rotate around rollers 16. As is common in belt CMP systems, a platen 14 is positioned under the pad 12 to provide a surface onto which a wafer will be applied using a carrier 18 (as shown in FIG. 1D). The pad 12 also contains a pad slot 12a so end point detection may be conducted as described in FIG. 1D.
FIG. 1D shows a typical way of performing end-point detection using an optical detector 20 in which light is applied through the platen 14, through the pad 12 and onto the surface of the wafer 24 being polished. In order to accomplish optical end-point detection, a pad slot 12a is formed into the pad 12. In some embodiments, the pad 12 may include a number of pad slots 12a strategically placed in different locations of the pad 12. Typically, the pad slot 12a is designed small enough to minimize the impact on the polishing operation. In addition to the pad slot 12a, a platen slot 22 is defined in the platen 14. The platen slot 22 is designed to allow the optical beam to be passed through the platen 14, through the pad 12, and onto the desired surface of the wafer 24 during polishing.
By using the optical detector 20, it is possible to ascertain a level of removal of certain films from the wafer surface. This detection technique is designed to measure the thickness of the film by inspecting the interference patterns received by the optical detector 20. Additionally, conventional platens 14 are designed to strategically apply certain degrees of back pressure to the pad 12 to enable precision removal of the layers from the wafer 24.
In typical end point detection systems such as shown in FIG. 1C, an optical opening is cut into a polishing belt. As shown in FIG. 1B, an optical opening is generally utilized within a polishing pad and a platen so a laser or light may be shined onto the wafer and a reflection may be received to determine the amount polished from the wafer.
FIG. 1E shows a dual graph 40 of end point detection data obtained from utilizing a broad spectrum of light end point detection that illustrates polishing distance detection. In an upper graph 41 showing the reflected light intensity, a curve 42 shows the intensity level of reflection for different frequencies of a light utilized for end point detection. The upper graph 41 has a vertical axis that indicates intensity and a horizontal axis showing frequency. The curve 42 with the upper graph 41 shows the differing intensity of light reflection from a wafer depending on the different frequencies of optical signals transmitted to the wafer. The intensities of light reflection as shown by the curve 42 is the optimal optical signal transmission through an optical window without any slurry on top of it. Unfortunately, when the light is blocked by slurry as occurs in prior art flat optical window systems, intensity of the light transmitted to the wafer and received back from the wafer by an optical detection unit is decreased (signal/noise decreases) as shown by a curve 44 which is a typical prior art profile curve. Therefore the curve 42 is not achieved by prior art systems when slurry accumulates in the polishing window.
Once a fourier transform 50 is conducted, a peak 46 and a curve 48 are shown in a lower graph 43 showing end point detection (EPD) intensity. The lower graph 43 has a vertical axis of intensity and a horizontal axis of thickness. The peak 46 of the lower graph 43 is produced by way of the fourier transform 50 of the curve 42, and the curve 48 is produced on the lower graph 43 by the fourier transform 50 of the curve 44. If an optical signal received by the optical detection is weak, as shown by curve 44, then the curve 48 is fuzzy and not as sharp as the peak 46 which results from reception of a strong optical signal by the light detection unit. Consequently, the curve 48 does not show as precise a film thickness polished as peak 46. Therefore, the stronger the optical signal received, the clearer measurement of film thickness that is made by the optical detection unit. Therefore, it is highly advantageous for a strong optical signal to be able to pass to the wafer or reflect from the wafer through an optical window to reach the optical detection unit.
FIG. 1F illustrates a prior art flat optical window system 60 for use during end point detection in a CMP process. In this example, a polishing pad 62 moves over platen 64 which in this example is a metallic table which may lend support to the polishing pad during the polishing action. A flat optical window 66 is attached to the polishing pad 62, and during polishing moves over a platen opening 70 which is generally a hole exposing the flat optical window 66 to an optical detector 72. Generally, flat optical windows of the prior art have a thickness of between 15 and 30 mils (a mil equals 1xc3x9710xe2x88x923 inch). As slurry 68 is deposited on top of the polishing pad 62, the slurry 68 accumulates in a polishing pad hole above the flat optical window 66. Unfortunately, the accumulation of slurry reduces reflection back of the optical signal to the optical detector 72, especially for shorter wavelength signals.
Unfortunately the prior art method and apparatus of end point detections in CMP operations as described in reference to FIGS. 1A, 1B, 1C, 1D, 1E, and 1F have various problems. The prior art apparatus also has problems with oxide removal where too much or too little may be removed due to inaccurate readings in optical endpoint detection resulting from accumulation of slurry in the flat optical window. Specifically, the accumulation of slurry often decreases the intensity of optical signal received by the optical detection unit from the wafer as shown in FIG. 1E. Because the prior art optical windows are configured to be flat in a polishing pad opening, slurry dispensed during CMP pools in the polishing pad hole. As more and more slurry flows into the polishing pad hole, more optical signal interference is created. This may significantly reduce wafer polishing accuracy and resultant wafer production reliability. Such a decrease in wafer polishing accuracy may serve to significantly increase wafer production costs. Consequently, these problems arise due to the fact that the prior art polishing belt designs do not properly control and reduce slurry accumulation on top of the optical window.
Therefore, there is a need for a method and an apparatus that overcomes the problems of the prior art by having a polishing pad structure that reduces slurry accumulation over an optical window that further enables more consistent and effective end point detection for more accurate polishing in a CMP process.
Broadly speaking, the present invention fills these needs by providing an improved optical window structure for polishing a wafer during a chemical mechanical planarization (CMP) process. The apparatus includes a new, more efficient, improved CMP pad with shaped optical windows that are more resistant to slurry accumulation and therefore increase reception of light intensity by an optical detection unit due to less slurry in an optical window hole. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, an optical window structure is provided. The optical window structure includes a support layer that has a reinforcement layer and a cushioning layer. In addition, the optical windows structure has a polishing pad which is attached to a top surface of the support layer. Furthermore, the optical window structure has an optical window opening and a shaped optical window. The shaped optical window at least partially protrudes into the optical window opening in the support layer and the polishing pad during operation.
In another embodiment, an optical window structure is provided. The optical window structure includes a support layer where the support layer has a reinforcement layer and a cushioning layer. The optical window structure also includes a polishing pad. that is attached to a top surface of the support layer and a flexible optical window, and the flexible optical window at least partially protrudes into an optical window opening in the support layer and the polishing pad when air pressure is applied to a bottom surface of the flexible optical window.
In yet another embodiment, an optical window structure is provided. The optical window structure includes a support layer where the layer has a reinforcement layer and a cushioning layer. The reinforcement layer is stainless steel and the cushioning layer is polyurethane. The optical structure also includes a polishing pad where the polishing pad is attached to a top surface of the support layer. The polishing pad is a polymeric material. The optical structure further includes a shaped optical window where the shaped optical window at least partially protrudes into an oval optical window opening in the polishing pad. A top surface of the shaped optical window is recessed between about 0.010 inch to about 0.030 inch below a top surface of the polishing pad, and the shaped optical window is one of a transparent material and a semi-transparent material.
In another embodiment, an optical window structure is provided. The optical window structure includes a support layer where the support layer has a reinforcement layer and a cushioning layer. The optical windows structure also includes a polishing pad where the polishing pad is attached to a top surface of the support layer. The optical window structure further includes an optical window opening and a shaped optical window. The shaped optical window at least partially protrudes into the optical window opening in the support layer and the polishing pad during operation. In this embodiment, the polishing pad is a polymeric material, the cushioning layer is a polymeric material, and the reinforcement layer is stainless steel.
The advantages of the present invention are numerous. Most notably, by constructing and utilizing a shaped optical window structure in accordance the present invention, the polishing pad will be able to provide more efficient and effective planarization/polishing operations over wafer surfaces (e.g., metal and oxide surfaces). Furthermore, because the wafers placed through a CMP operation using the shaped optical window structure are polished with better accuracy and more consistency, the CMP operation will also result in improved wafer yields. The shaped optical window structure of the present invention may utilize a shaped and raised optical window to keep slurry from accumulating on top of an area where optical signal may travel. Therefore, an optical detection unit utilized during end point detection may transmit and receive optimal optical signals through the shaped optical window to accurately determine the amount of polishing that has been completed in a CMP process.